1. Field of the Invention
The present invention relates to a method for enhancing adhesion of photo-resist to silicon nitride surfaces. More particularly, the present invention relates to a method for enhancing adhesion of a photo-resist layer to a silicon nitride layer on a semiconductor wafer, and therefore improves the critical dimension performance of a silicon nitride layer during a lithographic process.
2. Description of the Prior Art
Lithography technology is in wide use at various steps during an integrated circuit fabrication process, and plays a central role in microcircuit technology. Typically, five to twenty complete lithographic operations are required on each wafer. To transfer the desired pattern from a patterned photo-mask to a wafer, the wafer is coated with a light-sensitive photo emulsion, known as photo-resist. The photo-resist coating process consists of spinning the wafer at high speed after a small quantity of pre-filtered photo-resist has been placed on it. The photo-resist film thickness is inversely proportional to the square root of the spin rate; typically, spinning speeds range from 1000 to 6000 rpm and result in photo-resist films that are about 0.5 to 3 m thick. The photo-resist becomes degraded upon exposure to ultraviolet light. The photo-resist film must adhere sufficiently well to the underlying film so that it does not lift off during subsequent processing. This is usually not a problem, provided that appropriate procedures are used for surface preparation.
Extreme care must be taken to use clean, dry wafers to obtain good adhesion of the photo-resist. A dehydration bake process is thus routinely performed for a couple minutes to remove residual water. A dip in a coupling agent, well known as a priming process, just prior to photo-resist application, is then used to enhance adhesion of the photo-resist to the surfaces, especially silicon dioxide surfaces, of the wafer. Hexamethyldisilizane, commonly abbreviated as HMDS, is often used for this purpose. Other adhesion promotors which can be used include trichlorophenylsilane, trichlorobenzene, and xylene. The coupling agent can also be applied by vapor-plating, in a bath process. By virtue of HMDS, the surface energy of a silicon dioxide layer on the wafer can be adjusted to a level that is approximately equivalent to that of the photo-resist so as to enhance the adhesion.
However, adhesion of the photo-resist to some surfaces, such as silicon nitride surfaces, often presents a serious problem, especially when the line width is below 0.15 m. Please refer to FIG.1. FIG.1 is a cross-sectional diagram of defined gate electrodes 14 having a cap nitride layer 16 on each gate electrode 14 by virtue of a photo-resist layer 16 and HMDS 22 on a semiconductor wafer 10 after performing a dry etching process according to the prior art method. As shown in FIG.1, the semiconductor wafer 10 comprises a silicon substrate 12 and a plurality of gate electrodes 14 with a 0.15 line width defined on the silicon substrate 12. Each gate electrode 14 comprises a gate oxide layer 19 on the silicon substrate 12, a polysilicon layer 18 on the gate oxide layer 19, and a cap nitride layer 16 on the polysilicon layer. On each cap nitride layer 16 a photo-resist layer 20, acting as a hard mask during the dry etching process, is formed and HMDS 22 between the photo-resist layer 20 and the cap nitride layer 16 is applied according to the prior art.
Despite the application of the HMDS 22 to the surfaces of the cap nitride layer 16, the photo-resist layers 20 are still lifted off during a routine wet cleaning process that is performed before the dry etching process that is used to define the gate electrodes 14. This causes a pattern transfer failure. In addition, variations of the critical dimension (CD) performance are usually observed when using typical optical printing methods on the cap nitride layer 16. This occurs because, gradually, native oxidation of the surface of the cap nitride layer changes the optical properties of the surface, such as the reflection index, on the cap nitride layer 16. The problems of photo-resist patterning on silicon nitride layer are especially severe. Consequently, an economic and effective solution to the above mentioned issues must be found.